In single engine light aircraft, it is common that a number of the instruments are air pressure operated. These instruments may include a heading indicator, attitude indicator and turn and bank indicator. Applicant's U.S. Pat. No. 4,364,268 issued Dec. 21, 1982, is directed to apparatus for providing a redundant vacuum or air pressure source to provide for continued operation of the aircraft instruments in the event there is a failure of the primary air pressure pump. Such apparatus for providing a backup air pressure source includes a second pump connected to the instruments in such a manner that the second pump is operable in response to failure of the primary pump. In normal practice the backup pump is mounted in the engine compartment forward of the firewall.
Air pumps of the type employed to operate aircraft instruments commonly comprise impeller type pumps having an aluminum housing and a carbon impeller. The pump is constructed such that the planar end faces of the impeller are very closely spaced with respect to opposed planar faces of the aluminum housing. The aluminum forming the housing has a coefficient of expansion of 0.00124 inches per 100.degree. F. The carbon material commonly used in the construction of the impeller commonly has a coefficient of expansion of 0.00025 inches per 100.degree. F. Because of the differences in the rates of expansion of these materials, if the pumps are placed in a very cold environment, for example, -40.degree. C., the aluminum housing may contract sufficiently that the impeller will be immovable and the pump inoperable.
When an airplane is operated at high altitudes, the ambient temperature may be -40.degree. C. or colder, and it is possible that the secondary pump for the aircraft instruments may become so cold that the pump will become inoperable.